The metallurgical art in concentrating many mineral ores such as the metal sulfides, e.g., copper, lead, zinc, iron, molybdenum, nickel or combinations of these, and other mineral species, clay, coal, iron ore etc., has been confronted with the problem of dewatering ore concentrates obtained by conventional separation methods, in which the minerals are separated from the relatively worthless gangue by flotation in an aqueous medium. The resulting ore concentrates comprise a slurry or pulp, that is, a mixture of ore concentrates and water, which has included an undesirably great proportion of water. In the general art of mineral ore concentration this slurry or pulp is processed in a system comprising a thickener and either a rotary drum or leaf vacuum filter.
The slurry from the flotation process which contains thin solids, typically 15-25% by weight, is introduced to a thickener where the solids settle and become concentrated at the bottom. Flocculants are occasionally added to the slurry to facilitate this settling process. The thickener overflow consisting of a large volume of high quality water is returned to the mill water supply for further use. The thickened solids, typically 35-75% by weight, are then pumped to the filter where additional water is removed as a filter cake is built up upon the filter media. The filter cake is discharged as the product while the filtrate is returned to the thickener. However, in the filtration of ore concentrates without the use of filtration or drainage aid, the resulting filter cake is generally wet, plastic, sticky and difficult to handle.
In order to more fully describe the problem area to which the present invention is directed, a specific illustration of a particular operation might be helpful. Although a discussion of the operation and the problem specifically experienced in the copper recovery process follows, it should be understood that the similar problems are experienced in the concentration of any of the metal sulfides, lead, zinc, nickel, iron, molybdenum, combinations thereof or in the recovery of other mineral species, e.g., coal, clay, iron ore (iron oxide). A flow diagram of a typical filter plant is depicted by FIG. 1 of the drawings. The description following, together with the descriptive matter in the figure, makes further explanation thereof unnecessary.
Basically, the segment of the recovery process to which the present invention is directed is the filter plant. In standard copper mill applications, the copper concentrate which has been ground and floated away from the gangue materials is processed further through cleaner and recleaner flotation circuits. If a molybdenum (moly) recovery plant is not being used, the overflow from the recleaner circuit is pumped directly to the concentrate thickener. If a moly recovery plant is in use, the concentrate is processed further where the moly is floated and the copper is depressed and comes off as moly plant tailings which are then pumped to the concentrate thickener. In either case referring to FIG. 1, the stream containing the copper concentrate (1) arrives at the thickener (2) with a concentrate solids level of between 15-25%. The typical particle size of the solids expressed as percent which is retained on a 65 mesh screen when dry is 0%. Depending upon the individual plant operations, these solids are concentrated generally to a level between 40-70% and are then pumped through line (3) to one or more filters (5). There are a wide variety of filters in use in the industry; however, the more common units in use are rotary vacuum drum or disc filters.
The thickened concentrate slurry is contained in the filter tub (4). The system may be operated with an overflow or return to the thickener (6) so that the solids content in the circulating load may be controlled. When using a drainage aid to assist in the filtration operation, it is desirable to operate without an overflow so that all of the concentrate slurry entering the filter tub will be treated and none of the treated material will be returned to the thickener.
In the operation of the filter (5), the filter media (cloth) is immersed in the concentrate slurry for a period of 30-60 seconds under vacuum forming a cake. As the filter media (cloth) emerges from the surface of the concentrate slurry, the vacuum is continued so that the remaining moisture in the filter cake can be reduced to the lowest possible level. Time period for the drying cycle is typically between 40-70 seconds. The speed of most filters can be controlled so that cake formation and drying times can be adjusted to meet changing mill conditions. With the drum type filter, the filter media passes over a breaker bar (7) where the cake breaks up, falls vertically through chutes (8) onto a horizontal conveyor belt (9) and is taken to concentrate storage (10). In the disc type filter, the filter bags are generally expanded by air pressure to loosen the dried concentrate which either falls off, or if wet, is removed by scrapers. The vacuum in the filters is ideally maintained at 15-25" Hg; however, many of these plants, because of various reasons, run the vacuums in the range of only 10-12" Hg.
Those skilled in the art of filtration will recognize that there are a large number of variables which can effect the rate of filtration and there are many problems which are encountered in the operation of filter plants. The end result of the majority of these problems is usually a higher residual moisture level of the filter cake. It is not practical to attempt any sharp classifications concerning the relative significance of the residual moisture level to an individual mill operation, however, the following discussion of copper recovery operations will serve to illustrate some of the problems in handling wet filter cake at various residual moisture levels. In addition, high residual moisture imposes economic penalties in the form of shipping and drying costs which will become apparent in an ensuing section.
With a residual moisture level in the range of 17 to 23%, by weight, the filter cake is usually very thin, watery and will not discharge cleanly from the filter fabric. Further throughput of the filter can be seriously impaired at these moisture levels. A residual moisture level of between 14 to 17%, by weight, is fairly typical of many mills filtering copper ore concentrates. At this moisture level, the filter cake is still wet and sticky; however, it can be handled. A common problem encountered in mills operating within this moisture range lies in the stickiness of the wet filter cake. This property causes the cake to adhere to the filter fabric or conveyor belts where it eventually builds up. This build up of the abrasive concentrate is eventually forced into shaft or idler bearings seriously shortening their useful life. These problems require constant attention of Filter Operators or cleanup crews to assure that the automatic operation is continued with the least amount of interruption.
A residual moisture level between 11 to 14% in the discharged filter cake is a more common range encountered in the processing of copper ores. While still somewhat plastic, filter cake with this level of moisture can be handled on most plant filtration equipment. The most desirable range of residual moisture level in the discharged filter cake is between 8 to 11%. In this range, filter operational problems are minimized. The economic penalties associated with the shipping or drying of the wet filter cake are also minimized. With the present state of the art, it is impractical to reduce the moisture level of filter cake significantly below 8%, since the resulting losses from dusting during transportation or materials handling off-set the economic advantages of lower moisture levels.
When the filter cake moisture is above 14.0%, the cake is compressible and quite sticky. Filter cake with these moisture levels does not release cleanly from the surface of the fabric. On disc filters, the wet cake must be scraped off, which is only partly effective and seriously shortens the useful life of the filter bags. On both disc and drum filters, the material which sticks on the filter fabric and is not discharged returns to the filter tub and can increase the solids levels, particularly slimes, to an undesirably high level. With continued exposure to high vacuums in the filter cycle, the adhered material becomes embedded in the surface of the fabric, reducing its porosity and shortening its useful life.
Although attempts have been made to overcome the foregoing difficulties and numerous other operational problems in filter plants, none, as far as we are aware, prior to the instant invention, have been entirely successful, when carried into practice commercially on an industrial scale.
Stoneman et al, in U.S. Pat. No. 2,864,765, provides a very comprehensive description and discussion to the area to which the present invention is directed and teaches the use of a polyoxyethylene ether of a hexitol anhydride partial long chain fatty acid ester, functioning alone or as a solution in kerosene. While moderately effective in reducing the moisture levels of the resulting filter cake, these products are quite expensive in their own right and, in addition, require uneconomical feed rates to achieve the desired moisture levels. There are several additional deterents in reducing these teachings to practice on an industrial scale.
The compounds disclosed are essentially not adsorbed upon the solid surface of the ore particles and remain in the filtrate. As this filtrate is returned to the thickener, the concentration builds up over a period of time and several serious problems may result. With a number of ores these compounds can function as weak dispersants and seriously interfere with the basic operations of the thickener (which is to concentrate rather than disperse). An additional problem which results as the concentration of these compounds increases is that they are eventually discharged from the thickener overflow into the mill supply water where they may cause serious disruptions to the metallurgical recovery in the froth flotation circuits.
A second class of compounds, the sulfosuccinate esters, particularly di-2 ethyl hexyl sulfosuccinate, have also been used as dewatering aids in the filtration of mineral ore concentrates. While providing excellent performance as dewatering aids in some mills, these compounds are subject to similar problems indicated above in interfering with the thickener operations and disrupting metallurgical recovery in the flotation circuits. Those skilled in this art frequently resort to massive lime treatment of the thickener overflows in an attempt to hydrolyze the sulfosuccinate ester and prevent its interference in the flotation circuits. Published reports indicate this massive lime treatment is only partially successful. A further attendant difficulty with the use of the sulfosuccinate esters is their tendency to thin the filter cake which reduces the amount of concentrate solids picked up during each filtration cycle. This tendency can result in a marked decrease in the overall capacity of the filter plant.
In summary, the filter plant represents an important area of a copper mill. Because the total output of these mills normally passes through the filter plant any operational problems here can affect the total recovery process. With the tonnage which these mills generally process, this can represent a severe economic loss.